In diffusion bonding or welding, the bonding occurs by operation of short-range interatomic forces. When the materials to be bonded are brought close together under proper conditions of pressure and temperature, a migration of the atoms at the interface between the materials occurs and a bond is formed.
In order to create a diffusion bond, the surfaces of the parts to be bonded must be clean of contaminants, such as oxygen, which hinder the formation of the bond. Thus, the bonding process is often carried out in an inert, a reducing, or a vacuum environment. The furnace environment is typically established using one of two methods. The first method utilizes a stationary, hard shell furnace which is sealed from the ambient atmosphere so that the furnace can be evacuated to a desired subatmospheric pressure. The furnace is filled with a suitable inert or reducing gas after evacuation. The second method involves encapsulating the parts to be bonded in a thin sheet metal container or retort. The retort is fabricated by fusion welding and is evacuated through a small tube welded to the retort.
Diffusion bonding is usually performed at a temperature equal to about one-half of the melting point of the material being bonded. The use of such an elevated temperature serves two functions. First, the rate of atomic transfer of contaminants away from the workpiece surface increases as the temperature increases. Second, an elevated temperature allows for moderate deformation of the faying surfaces to enhance contact therebetween. Heat is typically applied to the workpiece in stationary, hard shell furnaces by electrical resistance elements, although induction heating has also been employed. If the parts to be bonded are encapsulated in a retort, the entire assembly is heated, for example, by electrical resistance heaters surrounding the retort.
A number of techniques have been used to apply pressure or a load to the faying surfaces, thereby bringing the surfaces sufficiently close together to effectuate bonding. A load is often imposed on parts encapsulated in retorts by applying a positive gas pressure to the exterior of the evacuated retort. A load can also be applied to the workpiece by a hydraulic press with heated platens, by a rolling mill, or by a mechanical clamping device. When diffusion bonding is carried out in a stationary or hard shell furnace, pressure is applied to the workpiece by press platens contained within the chamber or by differential thermal constraint. The use of press platens is favored, however, since loading by differential thermal constraint requires the use of special jigs or fixtures made from materials that have a low coefficient of thermal expansion relative to the workpiece.
The previous methods for diffusion bonding suffer from disadvantages which make their use impractical under certain circumstances. The use of a retort suffers in that the retort is not reusable, the method is difficult to use on large workpieces, and there is a danger of explosion upon heating due to an increase in pressure of residual gases inside the retort. Likewise, the use of a stationary furnace has previously suffered from the inability to inexpensively maintain an adequate vacuum or inert environment and difficulty in effectively and uniformly heating the workpiece. Since those problems become more pronounced as the size of the furnace increases, the hot zones of the known stationary furnaces have been relatively small.
Accordingly, it would be highly desirable to have a diffusion bonding furnace that provides improved heating of the workpiece and an improved sealing mechanism to retain the inert, reducing, or vacuum environment in the furnace during operation. Additionally, the furnace should be capable of applying a high load to the workpiece, provide a relatively large hot zone, and be relatively easy and inexpensive to operate and maintain.